The present invention relates to a progressive power spectacle lens with a dioptric power varying progressively between a distance portion and a near portion.
FIG. 17 is a front view (viewed from an object side) of a progressive power spectacle lens 1 for a right eye. The lens 1 includes:
a distance portion 2 having a dioptric power for distance vision at an upper area of the lens;
a near portion 3 having a dioptric power for near vision at a lower area of the lens; and
an intermediate portion 4 between the near and distance portions.
A dioptric power in the intermediate portion 4 progressively varies from the upper portion to the lower portion. Such a power is given by the asymmetrical shape formed on the front or rear surface, which is referred to as a progressive side surface.
A rectangular coordinate is defined by a fitting point O as an origin, a horizontal X-axis, and a vertical Y-axis. The fitting point O is the point on the progressive side surface of the lens 1 determined by a manufacturer as a reference point for positioning the lens in front of the eye.
The power of the progressive side surface varies along a main meridian MMxe2x80x2 that is a virtual centerline extending substantially along the vertical direction. Specifically, the main meridian MMxe2x80x2 is coincident with the Y-axis in the distance portion 2, while it is bent toward a nasal side in the intermediate portion 4, and extends vertically with being shifted toward the nasal side in the near portion 3 by an amount Xm.
The progressive power spectacle lens 1 must include surface astigmatism on the progressive side surface since the distance portion and the near portion, which have different dioptric powers, are smoothly connected. In particular, a zone along the main meridian MMxe2x80x2 is a center of a view field of a user, and accordingly, it is desirable that the astigmatism along the main meridian MMxe2x80x2 is minimized in order to provide a clear vision zone. The clear vision zone is a zone through which a user obtains a natural and comfortable view.
In one type of the conventional progressive power lenses, the main meridian MMxe2x80x2 is designed as an umbilical line along which a surface astigmatism has a value of zero.
Conventionally, a progressive power lens is designed with a surface performance evaluation of a progressive side surface (referred hereinafter as xe2x80x9ca surface evaluating designxe2x80x9d) to reduce complicated and expensive calculation work. The lens having the umbilical main meridian results in good performance in terms of the surface performance evaluation. However, the lens having a good surface performance does not always have a good transmission performance in a transmitting performance evaluation using the ray-tracing method. The transmission performance (which corresponds to a worn condition) is more important than the surface performance for actual products.
It should be noted that two types of astigmatism are used in the specification. xe2x80x9cA surface astigmatismxe2x80x9d is an absolute value of the difference between the dioptric power of the progressive side surface in a maximum curvature direction where the curvature has the maximum value, and the dioptric power of the surface in a direction where the curvature has the minimum value. The surface astigmatism is only determined by the shape of the progressive side surface. On the other hand, xe2x80x9ca resultant astigmatismxe2x80x9d is an astigmatism caused on a fundus of an eye through the lens.
When a progressive power spectacle lens is provided with a large base curve, the transmission performance is substantially coincident with the surface performance. This means that the good transmission performance lens can be designed by the surface evaluating design. However, the large curvature of the lens results in a heavy and thick lens.
Recently, a small base curve is generally required to obtain a light and thin lens even in the field of the progressive power spectacle lens. When the progressive power spectacle lens is designed so as to have a small base curve, the transmission performance is not coincident with the surface performance. That is, the lens having the umbilical main meridian results in insufficient transmission performance.
Japanese provisional patent publication Nos. SHO 59-58415, HEI 1-221722, HEI 8-136868 and HEI 4-500870 (the counterpart of PCT international patent publication WO91/01508) disclose the progressive power spectacle lenses that have non-umbilical main meridians. Although each of the publications teaches the surface astigmatism along the main meridian, none of the publications disclose the variation of the surface astigmatism along the horizontal direction.
It is therefore an object of the present invention to provide an improved progressive power spectacle lens, which has an enlarged clear vision zone with employing a small base curve.
For the above object, according to the present invention, there is provided a progressive power spectacle lens, which includes:
a distance portion having a dioptric power for distance vision;
a near portion having a dioptric power for near vision; and
an intermediate portion having a progressive dioptric power for vision at ranges intermediate between the distance and near portions;
wherein a predetermined surface astigmatism is provided on a main meridian, and the surface astigmatism decreases and then increases as the distance from the main meridian increases in a horizontal direction within the near portion.
The variation of the surface astigmatism is desirable to satisfy the condition (1) on at least one point in the range of xe2x88x9230 less than Y less than xe2x88x9215, and further to satisfy the condition (2) on at least one point in the overlapped range of xe2x88x9230 less than Y less than xe2x88x9215 and 3 less than |Xxe2x88x92Xm| less than 10, when a rectangular coordinate (unit: mm) is defined by a fitting point O as an origin, a horizontal X-axis and a vertical Y-axis;
AS(Xm, Y) greater than 0.2,xe2x80x83xe2x80x83(1)
and
AS(Xm, Y)xe2x88x92AS(X, Y) greater than 0.05,xe2x80x83xe2x80x83(2)
where
AS (x, y) is the surface astigmatism at the point (x, y), and
Xm is a displacement (i.e., a distance along the X-axis) of the main meridian from the Y-axis defined by Xm=f(Y).
Further, the maximum curvature direction xcex8(x, y) that is defined as an angle (unit: degree) with respect to the X-axis at the point (x, y) is desirable to satisfy the conditions (3) and (4);
xe2x88x9210xc2x0 less than xcex8(Xm, Y) less than 10xc2x0,xe2x80x83xe2x80x83(3)
and
60xc2x0 less than |xcex8(Xmxc2x110, Y)| less than 90xc2x0,xe2x80x83xe2x80x83(4)
where the surface astigmatism at the points satisfying the conditions (3) and (4) are larger than 0.2.